The present invention refers to a device for separating a surface layer of a liquid, including a container, which is intended to be submerged in the liquid and which includes: a bottom; a wall member, which has a centre axis extending substantially vertically when the container is submerged in the liquid, which wall member is connected to the bottom at a lower end of the wall member and has a variable length along the centre axis; and a separating member connected to an upper end of the wall member, which separating member has an opening, communicating with a space formed in the container, and an overflow portion, arranged, during separation of liquid from the surface layer having a free liquid surface, to be located at least partly below and adjacent to this liquid surface and to permit the flowing of liquid from the surface layer over the overflow portion through the opening into the space, in which a volume of separated liquid is collected, wherein the device also includes a discharge conduit, which extends from a part of the space, said part being filled with liquid during separation, and is arranged to discharge liquid from the space, and a floating member, which is connected with the separating member and which is arranged to float on the liquid located outside the container and to exert a lifting force upon the separating member in a direction prolonging the wall member.
There is at many different occasions a need of separating relatively lighter liquids from relatively heavier liquids.
It may for instance refer to different liquids, such as water, wash liquid or cutting fluid, which are used within industry and which are to be purified from relatively lighter impurities such as oil. In these cases, the liquid to be purified is most frequently located in a collecting tank or the like. The relatively lighter oil impurity will thereby to a large extent float on the surface of the relatively heavier liquid. If such oil impurities are permitted to be collected in the collecting tank, this may lead to health problems due to the aggressiveness of the included liquids, aggregation and growth of bacteria, and due to formation of mould. In addition, cutting fluids, which are not purified, are a problem both from an environmental and economical point of view, since they thus can not be reused. Other connections, where there is a need of this kind, may be cleaning of the sea or a lake from discharge of oil and from similar impurities floating on the surface, and purifying of water which has been contaminated by transformer oil.
U.S. Pat. No. 3,633,749 discloses such an initially defined device for separating of a surface layer. The wall member of the known device comprises a lower, outer cylinder and an upper, inner cylinder, which is partly displaceable in the lower cylinder. The two cylinders are connected to each other by a closed, flexible membrane, which permits said displacement. According to U.S. Pat. No. 3,633,749 the pressure difference on the membrane from the outside and the inside is utilised for providing a lifting force to the inner cylinder.
WO97/30821 discloses another device for separating a surface layer, which includes a floating body arranged in the space in the container and floating on the liquid in the container. When the liquid level in the container rises, the floating body will expand the bellow shaped wall of the container and close off the flow of liquid into the container. U.S. Pat. No. 5,693,218 discloses a similar device for separating a surface layer by means of a floating body floating on the liquid in the container.
Separating devices of these types are frequently used in different connections, which have different requirements thereto. Frequently, it is required that the separating device could be submerged in a tank or a tray for the liquid to be purified from relatively lighter impurities through an existing hole with a small diameter. The depth of the tank and the position of the free liquid surface within the tank may thereby vary within wide limits. This leads to a wish that such a separating device is to be have a small outer diameter and be provided with a wall element, the length of which along the centre axis is to be varied so much that the smallest length of the wall element frequently only should be a fraction of its maximum length.
Irrespective of if the floating member is arranged to mainly float on the liquid outside or the liquid within the container, the floating member has to be dimensioned in such a way that it is capable of lifting the separating member and prolonging the wall member by such a large force that it at maximum length of the wall element, when the downwardly directed, submerging force from the wall element upon the separating member is at the largest, is sufficient for lifting the separating member at need in such a way that its overflow portion is positioned above the free liquid surface and thus interrupts the flowing of liquid from the surface layer into the space in the container. At the same time, the separating device must be designed in such a way that the separating member is not lifted so much that the overflow portion is positioned above the free liquid surface when the liquid level in the space is below a certain level.